In a process for producing an aromatic dicarboxylic acid, such as terephthalic acid or naphthalene dicarboxylic acid, using, as the raw material, an aromatic compound having substituent groups of alkyl or partially oxidized alkyl, such as paraxylene or a dialkyl naphthalene, by subjecting it to a liquid phase oxidation with molecular oxygen, the thereby produced aromatic dicarboxylic acid will deposit out as crystals to form a slurry. Since the so-formed slurry contains the reaction catalyst, reaction solvent and so on, in addition to the crystals of the aromatic dicarboxylic acid, it is necessary to separate them to recover the crystals.
Explaining now the process by an example of production of terephthalic acid by a liquid phase oxidation of paradialkylbenzene, the thereby produced terephthalic acid deposits out of the mother liquor as crystals to form a slurry containing the terephthalic acid crystals. By recovering the crystals from, such a slurry, a crude terephthalic acid product is obtained. By dissolving the so-obtained crude terephthalic acid, subjecting the resulting solution to purification procedures, such as oxidation treatment, reduction treatment and so on, and then causing terephthalic acid to be deposited out, a crystal-containing slurry is obtained. By recovering the crystals from this slurry, purified terephthalic acid is obtained.
For recovering the crystals from the slurry, it has heretofore been practised in both cases, to subject the slurry to a solid/liquid separation using a filter or a centrifuge, followed by reslurrying the so-filtered crystals in a washing liquid and subjecting the slurry to a solid/liquid separation using a filter or a centrifuge and, if necessary, repeating such procedures many times. In such a technique, however, there is a problem that the process steps are complicate and the apparatus therefor becomes larger.
Therefore, a technique has been adopted as an alternative therefor, in which a series of process steps of pressure/suction filtration, washing and pressure/suction filtration are performed all at once on a rotary filter (Japanese Patent Kokai Hei-1-299618 A, corresponding to U.S. Pat. No. 5,093,001). This technique carries out a pressure/suction filtration, washing of the filtered cake, a pressure/suction filtration of the reslurry and removal or exfoliation of the filtered cake successively in order while rotating a cylindrical filter medium (filter cloth). If a rotary vacuum filter is employed for the rotary filter to effect a sucking filtration of wet cake in this technique, deposition of crystals would occur due to the condition of negative pressure on the filtrate side, resulting in a clogging of the filter. Therefore, the filtration and washing of the wet cake are realized in this technique by pressurizing the wet cake, in order to prevent any crystal deposition to occur.
In rotary filters however, a larger unit for gas circulation is necessary not only for sucking filtration but also for pressure filtration. Moreover, an efficient replacement of the reaction solvent may difficultly be achieved, since the circulation gas will pass through the wet cake after the gas has received the vaporized acetic acid, whereby acetic acid will remain in the crystals. Moreover, it is necessary to install a means for interrupting pressure in the passage for delivering the crystals, since the apparatus is kept pressurized in case the wet cake is subjected to a pressure filtration in a rotary filter.
The object of the present invention is to provide a process for producing an aromatic dicarboxylic acid which permits an efficient separation and washing of the product crystals by simple procedures using a simple apparatus without suffering from clogging of the filter and which can afford to recover the reaction solvent and the catalyst and to perform an efficient reaction solvent replacement as well.